<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">点我一下</button>
</body>
</html>
<script>
    var btn=document.getElementById('btn')
    btn.onclick=function(){
        btn.style.fontSize='100px'
    }
</script>